ABSTRACT
The effects of sludge retention time (SRT) on extracellular polymeric substances (EPS) content, the characteristics of sludge, the size distribution of sludge particles and the membrane resistance of filtration in the membrane bioreactor were studied. The results indicate that the EPS content is about 90 mg/g in the mixed liquor and 0.8 g/m2 on the membrane surface at 30 d of SRT. And EPS contents increase simultaneously with the extension of SRT, but EPS is hardly accumulated on the membrane surface. In addition, the electric charge on the surface of bacteria change and the proportion of hydrophilic group and hydrophobic group enhance with the rise of the EPS content, then the bacteria are transformed from the unstable (R) from to the stable (S) and the sludge is difficult to settle. The size-distribution profile of particles in the mixed liquor presents a bimodal shape at the long SRT and the peak values shift to the smaller range with SRT extension, at 30 d of SRT the average size of particles is about 14 microm and the two peaks occur at 0.5 microm and 16 microm, respectively. It shows that the content and the size distribution of small particles in the mixed liquor are crucial to the membrane fouling. It is found that it is suitable to control the SRT of membrane bioreactor below 120 times of the shortest generation-time of dominant bacteria (at the highest specific growth rate under the operation temperature) considering the membrane pollution and the characteristics of the mixed liquor.
